Heart rate is the number of times the heart beats per minute. Heart stroke volume is the volume of blood pumped during each heart stroke. Cardiac output is the volume of blood pumped in one minute and is generally considered to be the most significant gauge of cardiac fitness. Physicians must frequently rely upon such cardiac parameters to diagnose heart disease, to assess a patient's overall health, to determine the most appropriate method of treatment, and to quickly discover sudden lapses in cardiac performance.
The currently existing methods for measuring cardiac output and other cardiac parameters may be divided into two categories: invasive and noninvasive. The invasive methods require that a medical practitioner insert a measuring device into the patient's body, such as a catheter in the throat, and present numerous disadvantages to both patient and physician. The patient must often endure substantial pain and discomfort and the physician must perform a relatively complicated procedure and occasionally expose himself or herself to the risk of contact with infectious blood. The noninvasive methods currently in use represent a major advancement, but still have significant shortcomings. Most take measurements using ultrasound, phonocardiography, or electrical bioimpedance in order to calculate cardiac parameters.
The methods which employ bioimpedance measurement involve placing a plurality of electrodes on a patient's skin (predominantly in the thoracic region), generating a high frequency, low amplitude electric current from certain of the electrodes into the patient's body, measuring the changes in the electrical impedance of the patient's tissue over time, and correlating the changes in electrical impedance with cardiac parameters.
The manner of arranging the electrodes on the patient's body plays an important part in the relative accuracy of the ultimate cardiac parameter measurements. Due to various anatomical factors, electrodes must be placed over certain areas of the body to achieve optimum correlation between measured changes in bioimpedance and cardiac parameters. Many of the electrode configurations currently in use fail to adequately take into account the paths followed by the lines of electrical potential through the thorax and thus create a distortion in the cardiac measurement. Moreover, a few electrode arrangements require the use of band electrodes, e.g., influencing band electrodes A, B and measuring band electrodes C, D each having a width "n" (see FIG. 1). These band electrodes typically wrap around a patient like a belt and further limit access to the patient, an especially undesirable condition curing reanimation procedures. The movements associated with respiration also make band electrodes very inconvenient when placed on the neck and chest.
Perhaps the most significant problem with the presently existing bioimpedance methods in the imprecise mathematical derivation of cardiac parameters from bioimpedance measurements. The ventricular ejection time (VET) is a measurement of the time between the opening and closing of the aortic valves during the systole-diastole cycle of the heartbeat and it must be calculated as an intermediate step in determining cardiac stroke volume. The prior art does not teach a method for determining ventricular ejection time with sufficient accuracy. Furthermore, the prior art fails to account for the fact that VET is not a single event. In reality, there is actually a left VET and a right VET. It has been shown that the time-derivative impedance signal is actually proportional to the peak aortic blood flow ejected by the left ventricle. The measurements of left VET and right VET for most patients are generally very close, but even slight differences between them can create errors in bioimpedance readings under the methods currently in use.
Furthermore, the classic algorithm for ejection start time is elaborate, and works well only for healthy patients at rest. It is not accurate for patients under physical training or other physical stress, or for critically ill patients, such as those typically in intensive care units.
The conventional equation for deriving stroke volume from bioimpedance signals has become known as the Kubicek equation and is given as follows: EQU SV=R(L/Z.sub.0).sup.2 .multidot..DELTA.Z
where SV is heart stroke volume, R is blood resistivity, L is the distance between the inner and outer voltage sensing electrodes, Z.sub.0 is the mean thoracic impedance determined from the inner voltage sensing electrodes, and .DELTA.Z is the impedance change due to blood influx. Kubicek's estimation of this value is EQU .DELTA.Z=(VET).multidot.(dZ/df).sub.max
where VET is the combined left and right ventricular ejection time, and (dZ/dt).sub.max is the maximum negative slope change of the time-differentiated impedance signal. Most bioimpedance cardiac monitoring systems use some form of the Kubicek equation.
Without further refinement, however, the Kubicek equation frequently given inaccurate measurements. This is due in part to the fact that both ventricles contribute to impedance changes, and so Kubicek's calculated ejection time (VET) cannot be associated with a particular, specifically the left, critical ventricle. Concurrently, Kubicek's .DELTA.Z estimation becomes invalid when strong left-right ventricles asynchronism is observed. As a result, Kubicek's SV calculation is often proportional to, but not equal to, the actual heart stroke volume and must therefore be multiplied by some correlating constant. In addition, the prior art does not disclose a method for adjusting R in accordance with the fluctuation of a patient's hematocrit (red blood cell count). The adjustment of R is especially important in patients undergoing blood infusion.
Many of the methods for bioimpedance cardiography require that the patient hold his or her breath during each measurement because respiration causes interference in the bioimpedance signal. Such methods are inconvenient for some patients and completely useless for other patients who are unconscious or otherwise unable to hold their breath. Some of the more recent methods include signal processing capability to enhance the signal, to identify the effects of respiration, and to eliminate defective signals to that errors are not introduced into the final calculations. Effective signal processing is generally the key to insuring accuracy in bioimpedance cardiography and improvements in this are can represent significant advances in the art.